Imaging performed from an aircraft or satellite has specific constraints which do not exist in other domains such as photography intended for the general public. These constraints include such issues as:                the need to use an image-capturing instrument such as a telescope, at long observation distances,        having a good resolution, meaning a fine resolution, in spite of the very great image capture distance,        reducing the weight and size of the image-capturing instrument, which depends to a large extent on the dimensions of the optical components used for the image-sensing optical system,        the interfering movements of the aircraft or satellite which are transmitted to the image-capturing instrument,        except for geostationary satellites, the moving of the portion of the Earth's surface which is being photographed, relative to the aircraft or satellite, and        the need to reduce the amount of data transmitted to the Earth for the same captured image.        
Traditionally, the following parameters are used to quantify the performance of an imaging system:                the resolution, which is also referred to as Ground Sample Distance (GSD), for an imaging system carried on board an aircraft or satellite. This is the minimum distance there must be between two points, on the portion of the Earth's surface which is being photographed, for these points to appear separately in the captured image. It is equal to the size of the photodetectors of the array concerned, divided by the focal length of the image-sensing optical system which is used, multiplied by the distance between the object and the image: GSD=(p/Dfoc)×H, where p is the pitch between photodetectors, Dfoc is the focal length, and H is the distance between the object and the image;        the Nyquist frequency, which is denoted νN and which is equal to the inverse of twice the pitch of the sampling conducted by an array of photodetectors when capturing an image of a viewed object. In other words: νN=1/(2×GSD) in the object space, or νN=1/(2×p) in the image space. The Nyquist frequency therefore only depends on the size of the photodetectors of the array and on the focal length of the image-sensing optical system;        the modulation transfer function, which is referred to as MTF and characterizes the variation in contrast between the image and the photographed object, when this object has a spatial variation in light intensity. The modulation transfer function depends on the spatial frequency of the variation in light intensity of the object, and on the wavelength of the radiation detected; and        the cutoff frequency of the image-sensing optical system, which is denoted νC and which is the spatial frequency of the variation in light intensity of the object for which the modulation transfer function is reduced to zero, for the radiation wavelength which is detected.        
The modulation transfer function results from several contributions, including:                a contribution from the image-sensing optical system, which depends on the light diffraction caused by its pupil and on optical aberrations of this instrument;        a contribution from the photodetectors, which depends on the individual performances of each photodetector and on crosstalk that may arise between photodetectors; and        a contribution from the movements and vibrations of the aircraft or satellite during the exposure period when capturing an image.        
However, the cutoff frequency νC is a characteristic of the image-sensing optical system alone, dependent on the pupil diameter, the focal length, and the wavelength considered. Given that the cutoff frequency depends essentially on the diffraction of radiation by the pupil of this optical system, it decreases as a function of the wavelength of the radiation transmitted by this optical system. As a result, when an image is captured which is formed from radiation containing several wavelengths, the resulting cutoff frequency νC is the one corresponding to the longest of these wavelengths. Only the object's variations in light intensity which have spatial frequencies less than the cutoff frequency νC are transmitted by the image-sensing optical system.
In general, an image-capturing instrument intended for use on board an aircraft or satellite, most often the telescope itself, is designed based on a desired resolution value. This resolution value determines the pair of parameters consisting of the focal length of the image-sensing optical system and the size of the photodetectors in the image detection array which is used. Then the dimensions of the entrance pupil of the image-sensing optical system are determined, with other parameters, so that the light diffraction caused by this pupil is compatible with the desired resolution value. To this end, the pupil is sized so that the cutoff frequency νC is greater than the Nyquist frequency νN.
Many imaging systems already exist for capturing an image of a portion of the Earth's surface with color information. Usually, several images of the same portion of the Earth's surface are captured through filters which correspond to different colors, then the images which were captured for these colors are superimposed to obtain a reconstructed polychromatic image. In the polychromatic imaging technique known as pansharpening, one of the filters is said to be wide band, or panchromatic, because it contains all the respective colors of the other filters. In addition, the image corresponding to the panchromatic filter is captured with a finer resolution value than those of the monochromatic images. This panchromatic image therefore contains the structural information of each of the other colors. The resolution of the reconstructed polychromatic image then corresponds to the smallest of the individual resolution values of the images which are superimposed, meaning the finest resolution, which is that of the panchromatic image. The image-capturing instrument, particularly its pupil diameter, is then sized according to this smallest value of the resolutions of the individual captured images. The dimensions are therefore excessive for the monochrome images which have larger individual resolutions, meaning coarser resolutions. In addition, the amount of data corresponding to the reconstructed polychromatic image is equal to the sum of the amounts of data for each individual image which is captured, multiplied by the number of filters used. The total amount of data which corresponds to a reconstructed polychromatic image is therefore increased.
In photography intended for the general public, which is distinct from photography from an aircraft or spacecraft for the reasons listed above, a trichromatic image is captured in a single exposure. To this purpose, each photodetector in the array is individually combined with a color filter, which may be blue, green, or red. When the same number of photodetectors is dedicated to each filter color, the resolution of the camera corresponds to that of the array for each color.
Document U.S. Pat. No. 3,971,065 proposes a specific assignment of photodetectors in the image detection array to the colors blue, green and red: one out of every two photodetectors is associated with the green filter, one out of every four photodetectors is associated with the blue filter, and one out of every four photodetectors is associated with the red filter. The resolution of the image associated with the green color therefore corresponds to the pitch of the array used, multiplied by √{square root over (2)}, while the resolutions associated with the colors blue and red each correspond to twice the pitch of the array used. The color green is thus chosen in order to reduce the value of the resolution of the reconstructed polychromatic image. As the color green is close to the maximum spectral sensitivity of the human eye, the polychromatic image reconstructed by superimposing the three-color images has substantially the resolution of the color green.
Lastly, document US 2009/0051984 describes an improvement to the image capture method of U.S. Pat. No. 3,971,065. This improvement consists of adding photodetectors which are dedicated to capturing an image across all wavelengths of visible light, or panchromatic, in addition to monochrome images which are captured by the photodetectors associated with the green, blue and red filters.
Under these conditions, one object of the invention consists of reducing the dimensions of an image-capturing instrument which is carried on board an aircraft or spacecraft, without reducing the resolution of polychromatic images obtained with this instrument. This object particularly concerns the telescope, which is generally the heaviest and most voluminous part of the image-capturing instrument.
Another object of the invention consists of improving the apparent resolution of a polychromatic imaging system which is used to capture images of the Earth from an aircraft of spacecraft, without increasing the dimensions of the pupil of the image-capturing instrument in comparison to known imaging systems.
Yet another object consists of improving the rendering quality of a polychromatic image obtained from an aircraft or a spacecraft.
Yet another object consists of improving the modulation transfer function of an imaging system adapted to capture images of the Earth from an aircraft or spacecraft, for a fixed dimension of the pupil of the image-capturing instrument that is used.
Another last object consists of reducing the amount of data to be transmitted for a polychromatic image, while the resolution value for this polychromatic image remains the same.